The present invention relates to a magnetic memory. The invention relates particularly to a magnetic memory of a domain wall motion type.
Magnetic memories, in particular, magnetic random access memories (MRAMs) are nonvolatile memories capable of attaining high-speed operation, and rewriting data unlimited times. MRAMs have partially started to be put into practical use, and have further developed to make the multiusability thereof higher. MRAMs each make use of a magnetic body as a memory element, and memorize a data corresponding to the direction of the magnetization of the magnetic body. In order to write a desired data into the memory element, the magnetization of the magnetic body is switched to a direction corresponding to the data. As the method for the magnetization-direction-switching, many methods are suggested. The methods are common to each other in that an electric current (hereinafter referred to as a “writing current”) is used. To put MRAMs into practical use, very important is how much the writing current can be made small.
Non-Patent Document 1 (N. Sakimura et al., “MRAM Cell Technology for Over 500-MHz SoC”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 42, NO. 4, pp. 830-838, 2007) states that by decreasing the writing current given to MRAMs to 0.5 mA or less, the cell area thereof is made equivalent to that of the existing consolidated SRAMs.
The most general one out of methods for writing data into an MRAM is a method of arranging an interconnect around the magnetic memory element, causing a writing current to flow into this interconnect to generate a magnetic field, and switching the magnetization direction of the magnetic memory element by effect of the magnetic field. According to this method, in principle, data can be written in a period of 1 nanosecond or less. Thus, this method is suitable for realizing a high-speed MRAM. However, a magnetic field for switching the magnetization of a magnetic body about which thermal stability and disturbed magnetic field resistance are certainly kept is generally about several tens of oersteds. In order to generate such a magnetic field, it is necessary to use a writing current of about several milliamperes. In this case, it is unavoidable to make the area of the chip (concerned) large. Moreover, the consumption power required for the writing is also increased. For these reasons, MRAMs are poorer in competitive power than other random access memories. Furthermore, when such elements are made fine, the writing current therefor is further increased. Thus, MRAMs are unfavorable from the viewpoint of scalability.
In recent years, in order to solve such problems, the following two manners have been suggested.
The first is the spin torque transfer manner. According to the spin torque manner, in a stacked film composed of a fist magnetic layer having reversible magnetization and a second magnetic layer coupled with the first layer and having a fixed magnetization direction, a writing current is caused to flow to the first and second layers across these layers. At this time, conductive electrons the spins of which are polarized interact with localized electrons in the first magnetic layer, whereby the magnetization of the first magnetic layer can be reversed. When the written data is read, use is made of magnetoresistance effect generated between the first and second magnetic layers. Accordingly, the magnetic memory element in which spin torque transfer is used is a two-terminal element. Spin torque transfer is caused when the current density in this element is some value or more; thus, as the element size is smaller, the current required for writing is made smaller. In other words, it can be mentioned that the spin torque transfer manner is excellent in scalability. In general, however, an insulating layer is laid between the first and second magnetic layers; thus, when a data is written, it is indispensable to cause a relatively large writing current to flow through this insulating layer to the magnetic layers across the layers. As a result, the manner has problems about rewriting resistance and reliability. Moreover, the writing current path thereof is identical with the reading-out current path thereof, so that an incorrect data may be unfavorably written at the time of readout. As described herein, several barriers rise up against the practical use of the spin torque transfer manner although this manner is excellent in scalability.
The second is the current driven domain wall motion manner. MRAMs using current driven domain wall motion is disclosed in, for example, Patent Document 1 (Japanese Unexamined Patent Publication No. 2005-191032). In an MRAM of a general current driven domain wall motion type, a magnetic layer having reversible magnetization (data memorizing layer for memorizing data) is laid, and (partial) magnetizations at both ends of the data memorizing layer are fixed to be substantially antiparallel with each other. By such a magnetization arrangement, a domain wall is introduced in the data memorizing layer. As reported in Non-Patent Document 2 (Non-Patent Document 2: A. Yamaguchi et al., “Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires”, PHYSICAL REVIEW LETTERS, VOL. 92, NO. 7, 077205, 2004), when a current is caused to flow into a domain wall along a domain-wall-penetrating direction, the domain wall is moved into the direction of the conductive electrons (concerned). Accordingly, by causing a writing current in the in-plane direction to flow into the data memorizing layer, the domain wall is moved into a direction corresponding to the current, so that a desired data can be written. When the data is read out, a magnetic tunnel junction containing a region where the domain wall is moved is used to read out the data on the basis of magnetoresistance effect. Accordingly, the magnetic memory element in which current driven domain wall motion is used is a three-terminal element. In the same manner as spin torque transfer, current driven domain wall motion is also caused when the current density in the element is some value or more. It can be therefore mentioned that the current driven domain wall motion manner is excellent in scalability. Additionally, in the current driven domain wall motion manner, no writing current flows into the insulating layer, and further the writing current path is different from the reading-out current path. Accordingly, the above-mentioned problems caused in the spin torque transfer manner are solved. Non-Patent Document 2 described above reports that the current density necessary for current driven domain wall motion is about 1×108 A/cm2.
Non-Patent Document 3 (S. Fukami et al., “Micromagnetic analysis of current driven domain wall motion in nanostrips with perpendicular magnetic anisotropy”, JOURNAL OF APPLIED PHYSICS, VOL. 103, 07E718, 2008) states the usefulness of perpendicular magnetic anisotropic material. Specifically, it is made evident through micromagnetic simulation that when a data memorizing layer in which domain wall motion is caused has perpendicular magnetic anisotropy, the writing current therefor can be sufficiently decreased.
Patent Document 3 (International Publication WO/2009/001706) discloses a magnetoresistance effect element making use of a magnetic body having perpendicular magnetic anisotropy, and an MRAM having this element as a memory cell. FIG. 1 is a sectional view that schematically illustrates an example of the magnetoresistance effect element disclosed in this publication. The magnetoresistance effect element has a magnetic memory layer 110, a spacer layer 120, and a reference layer 130.
The magnetic memory layer 110 is made of a ferromagnetic body having perpendicular magnetic anisotropy. The magnetic memory layer 110 has a first magnetization fixed region 111a, a second magnetization fixed region 111b, and a magnetization liberalized region 113. The magnetization fixed regions 111a and 111b are arranged at both sides of the magnetization liberalized region 113, respectively. The magnetizations of the magnetization fixed regions 111a and 11b are fixed in directions reverse (antiparallel) to each other. As illustrated in FIG. 1, for example, the magnetization direction of the first magnetization fixed region 111a is fixed to +z direction; and that of the second magnetization fixed region 111b, to −z direction. The magnetization direction of the magnetization liberalized region 113 is reversible through a writing current flowing from any one of the magnetization fixed regions to the other, so as to be +z direction or −z direction. Thus, in accordance with the magnetization direction of the magnetization liberalized region 113, a domain wall 112a or 112b is formed inside the magnetic memory layer 110. A data is memorized as the direction of the magnetization of the magnetization liberalized region 113. The data may be regarded as being memorized as the position of the domain wall 112 (112a or 112b). The reference layer 130 which is made of a ferromagnetic body having a fixed magnetization direction, the spacer layer 120 which is a nonmagnetic layer (insulating layer), and the magnetization liberalized region 113 form a magnetic tunnel junction (MJT). Any data is read out as a large or small resistance value of the MJT.
This document, Patent Document 3, discloses that when the magnetic memory layer 110 has perpendicular magnetic anisotropy, the writing current can be decreased.
Out of material objects having perpendicular magnetic anisotropy, stacked films made of Co and Ni, of Co and Pt, and of Co and Pd, respectively, are generally known. A Pt or Pd layer may be laid on or beneath the stacked film of Co and Ni, or interposed between two layers thereof. The material used for hard disk media is, for example, CoCr based alloy, or a mixture of CoCr based alloy, and SiO2 or TiO2. The material used for magnetooptical recording media is, for example, TbFeCo.